The integration of such a bag in a closed system with a view to separating the blood into its components is known, in particular from the document U.S. Pat. No. 4,767,541.
To this end, the system typically comprises means of taking blood, a primary bag intended to collect the blood and, connected to this by means of tubes, at least one filter and at least one secondary bag intended to receive the filtrate.
The use of such a system involves successively a centrifugation step for separating the blood into its various components, namely plasma, white corpuscles or leukocytes, and red corpuscles, and a step of filtration of at least one component of the blood, in order to eliminate the leukocytes. It is in fact known that leukocytes are undesirable in blood components to be transfused.
During the centrifugation step, it is necessary to introduce into the centrifugation bowl not only the various flexible bags but also the filter.
However, it proves that, during this step, the filter may be damaged, because of the acceleration and the centrifugation speed: it may in fact bend, thus creating preferential passages for the blood, or may break, which consequently makes the filtration inoperative.
In addition, and in particular when the filter has a rigid enclosure, the centrifugation thereof conjointly with the flexible bags and the tubes may, through friction, cause damage to these. In some cases, this damage may cause piercing of a bag and/or a tube, which makes the system unusable for the separation of the blood into its components.
To remedy these problems, various systems have already been proposed. These can be classified into two categories:                systems requiring a particular arrangement of the centrifugation bowl, for example by inserting in the bowl a rigid support intended to accept the filter during the centrifugation step and therefore to separate the filter from the other components of the system;        systems having recourse to means external to the centrifugation bowls, namely, for example, means of associating the filter on the bowl.        
The systems in the first category have in particular the drawback of requiring a particular type of support for each centrifugation bowl. In particular, the presence of the rigid support inside the bowl limits the space available for the system.
It is known that it is possible to provide, for the systems in the second category, either association means integrated in the filter or external to it. In the first case, the production of the filter is made more complicated. In the second case, adhesive strips are in particular used which provide unreliable association.
Moreover, it is known from the document U.S. Pat. No. 5,836,934 that a case can be associated on a bag in order to house a filter during the centrifugation. This solution has in particular the drawback of not being simple to implement in that it requires the production of a specific case which must be associated with the bag during a specific step. In addition, the association is effected on one face of the bag which, during filling of the bag, deforms. Thus a loss of reliability of the association may be caused, notably during the high forces which are caused by the centrifugation.